This invention relates to communications systems in general, and in particular to signaling methods and apparatus in systems for transmitting a plurality of information signals over a single transmission link.
Modern communications systems require the transmission of ever increasing amounts of information at relatively high rates of reliability. Such communications, however, are often subject to interference from both natural and manmade phenomena. For example, noise, multipath effects, fading bandwidth, and electro-magnetic interference decrease the probablity that a transmitted signal will be received correctly.
Typically, reliability and speed of communication are inversely related. The more information which is sent in any given time in a particular bandwidth reduces the reliability that the information will be received correctly. Consequently, there has come about an interplay of methods which sacrifice transmission speed for reliability, or vice versa.
Several techniques are known to be used to increase reliability, albeit at the cost of some transmission speed. In one known method, information to be transmitted is redundantly communicated, the same message being transmitted repeatedly a predetermined number of times. In another method, transmitted data are sent along with a mathematically derived correction code to identify and rectify, if needed, a limited number of possible errors in the transmitted information. It is also known to first transmit a known reference signal followed by an information signal. The receiver can evaluate the reference signal to "sound" the condition of the transmission media, it usually being assumed that the reference signal will experience the same types and amount of interference as the subsequent information signal.
As the need for high speed communications has grown, methods have been implemented which transmit more than one bit of information at a time on a signal transmission medium. For example, in a typical phase shift keyed (PSK) system, a carrier wave is modulated in phase (quadraphase). Similarly, systems are known in which a plurality of carrier waves are modulated simultaneously by a string of information bits, the plurality of modulated signals then being combined and transmitted as a single signal. In such systems, the receiver separates the composite signal by filtering or other means, demodulates each of the carrier waves, and retrieves the informational contents.
In such systems it is known to transmit reference signals along with the informational signals and to use the reference signals to decode the informational signals. For example U.S. Pat. No. 3,290,440 to Easton et al. discloses a data transmission system in which two information signals and two reference signals are transmitted simultaneously. The informational signals are decoded by comparing each to one of the references and by comparing the two references to themselves.
In the patent to Downey et al. (U.S. Pat. No. 3,404,229), a plurality of data signals each modulated on different carrier frequencies are combined with three reference signals and the composite signal is transmitted. The receiver utilizes the reference signals to determine the interference of the transmission medium and to subtract the effects of the interference from the data signals. In such systems, the reference signals are each transmitted at carrier frequencies, which are different from each of the frequencies of the data signals. Such systems assume that the interference, i.e., the noise, jitter and other errors are uniform for both reference and data signals. However, it is known that interference does not have the same effects on all frequencies in many instances.
It is therefore an object of the present invention to obviate many of the problems of known systems and to provide a novel communication system of improved transmission speed and reliability.
It is another object of the present invention to provide a novel communication system in which data signals are determined with reference to the interference effects measured at the same frequency as the data signal.
It is yet another object of the present invention to provide an improved signalling method for multiple carrier data transmission systems.
It is still a further object of the present invention to provide a novel communication system whereby transmitted data may be decoded with reference to channel interference occurring prior to, concurrent with, and/or subsequent to the data being decoded.
These and other objects and advantages of the present invention will become apparent to one skilled in the art from the claims and from a perusal of the following detailed description when read in conjunction with the attached drawings.